Move GL's SkSL::Compiler to the GPU (like all other backends)

This was the only backend that didn't store the compiler on the GrGpu,
and also the only one that did lazy-instantiation. Trying to standardize
this code a bit.

Change-Id: Ibdd1bcc2dc9c3756b46a4c6f0543b5bb20fe135d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/337716
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
7 files changed